1. Field of the Invention
The present invention relates to optical fiber communications methods, systems and terminals for use in such systems and to cellular and radio distribution points.
2. Description of the Related Art
The use of analog optical fiber transmission links to distribute cellular and other radio communications signals from a central location (where the radio base stations are situated) to one or more remote locations (where the antenna points are situated) is a well established technique commonly known as “radio over fiber.” Radio over fiber makes use of the broad bandwidth and low attenuation characteristics of optical fiber, which allows these systems to be deployed over very long spans for applications such as shopping malls and airports in the case of cellular radio. The remote antenna units in these systems comprise an optical transmitter (laser), an optical receiver (photodiode) and various electrical components such as amplifiers and filters. Most of these components are active and therefore need an electrical power supply in order to function. This electrical power supply is either provided locally (for example through a tap to a power outlet) or centrally using copper conductors in the cable linking the remote antenna units to the central unit where the electrical power supply is located (power over copper).
There are drawbacks to both ways of providing remote electrical power. Local electrical power supplies can be expensive or impractical to install, depending on the circumstances of the remote locations. Composite cable (containing mixed fiber and copper) is not a preferred cable type within the cabling industry and also has a relatively short reach capability due to ohmic losses in the copper. Furthermore, there are special situations, such as hazardous explosive environments and military applications, where electrical isolation and/or the avoidance of radio frequency interference or other undesired emissions from copper cable are critically important.
There are two approaches for eliminating a remote electrical power supply. The first approach is to have a remote unit that does not require a power supply at all. U.S. Pat. No. 6,525,855 discloses a remote antenna unit that requires no electrical power supply. The remote antenna unit relies, instead, on the use of an unbiased electroabsorption modulator as an optical detector and as an optical modulator, i.e., as an optical transceiver that requires no electrical power supply. If no electrical amplifiers or other active components are used in the remote antenna unit, then no electrical power supply is required at all.
This approach, however, has limited appeal in certain situations. First, only low radio frequency power is available, which limits the radio range to a few meters, depending on radio system and propagation environment. Second, electrical amplifiers are generally required in the remote antenna unit in order to boost the radio frequency power to levels required for acceptable radio range (typical amplifier gains required are in the range 20-30 dB depending on the application).
The second approach for eliminating a remote electrical power supply is to provide power through the optical fiber. “Power over fiber” is a technique that uses a high power laser at a central location, a photovoltaic converter at a remote location and an optical fiber linking the two sites for transmission of the optical power. The photovoltaic converter provides an electrical power output from the optical power input. For example, Banwell et al., “Powering the fiber loop optically—a cost analysis”, J. Lightwave Tech., vol.11, pp. 481-494, 1993 discloses the use of power over fiber to provide power to a subscriber's telephone equipment. Aside from this limited application, Banwell only considers power over fiber to be practical for low data rate telephony.
Carson, “Modular photonic power and VCSEL-based data links for aerospace and military applications”, proc. IEEE Aerospace Conference, vol.3, pp. 197-210, 1997, discusses power over fiber in the context of aerospace and military applications. In such applications, electrical shielding of the remote module is typically required. The remote unit disclosed by Carson, for example, is in a Faraday Cage Wall. To allow for such electrical shielding, the remote unit receives its power over fiber with the use of a photovoltaic converter. Carson explains that the use of photovoltaic converter demands low power consumption which, in turn, requires low data rate. The disclosed design is thus limited to ultra-low power consumption and low data rates under 10 kb/s. Carson notes that such a design is applicable when the photonic channels are not required to transmit analog radio frequency data.